Why Are We Overweight? Part 1
In 2006, University of North Carolina professor Barry Popkin reported to the International Association of Agricultural Economists that for the first time, overweight people worldwide outnumbered those who were starving. Now, if you’re a glass-half-full person, you might see this as good news because world hunger is a serious problem and this suggests a shift away from that terrible scourge. However, the pessimist would say that the net effect does not represent an improvement in the state of the world because being overweight is associated with myriad health problems including cardiovascular disease and cancer. Consequently, confirmation that almost one of six people is overweight or obese cannot be looked at as anything but a sobering revelation.
For those who spend time in public and keep their eyes open while doing so, the overweight epidemic should come as no surprise. However, a conclusion as to why this epidemic exists is more difficult to reconcile. In fact, a recent article in Exercise and Sport Sciences Reviews by Professor Katarina T. Borer suggests that even experts have been missing the boat on this one. Professor Borer explains how mechanisms that have long been thought to regulate how much we eat and how active we are might not operate in the manner that has been proposed. Specifically, our views regarding the roles of hormones that have been implicated in the process might need significant revising. However, before I detail her findings, some background information is required.
One of the first things that a student of physiology learns is the concept of homeostasis. Simply stated, homeostasis is synonymous with normalcy and the living organism seeks to maintain this status by enacting every adaptive mechanism it has at its disposal. For example, if your neuromuscular and musculoskeletal systems are used to carrying groceries that weigh 10 pounds during your weekly trip to the supermarket, entering a weight room and lifting a 15-pound dumbbell will be interpreted as above and beyond the normal call of duty and, therefore, homeostasis will be disrupted and your systems will have to adapt. Specifically, their level of function will be enhanced such that a similar stress in the future will no longer rock the boat. Obviously, simply lifting 10 pounds in the gym would not elicit this response and, indeed, if you started lifting five pounds and got your spouse to carry the groceries, your systems would adapt in the opposite direction.
In addition to providing the basis for why we should exercise, homeostasis has also been cited as the determinant of how much body fat we accumulate. This belief dates back to research performed by E.F. Adolph in 1947 and is based upon the notion that how much we eat and how physically active we choose to be should wind up balanced such that weight stability is maintained. Subsequent refinement has led to a contemporary view of the homeostatic control of energy regulation based upon the influence of circulating hormones. For example, insulin secreted by the pancreas and leptin secreted from fat cells each circulate and enter the brain at levels proportional to body fat content and when either of these are administered directly into the brain of animals, a sustained inhibitory influence on feeding is observed. This means that there should be a desire to eat less when an energy surplus is in effect (i.e., when fat cell mass increases because energy intake exceeds expenditure) and vice versa. Furthermore, in addition to appetite and satiety, our resting metabolic rate and level of spontaneous physical activity should also adjust according to how much body fat we carry. Collectively, this means that a ‘set point’ should exist such that our body weight should not deviate from a narrow range determined mostly by genetics.
To put this into perspective, consider how homeostatic mechanisms function to maintain body temperature. If you happen to live in a place where the winters are cold, you could easily find yourself in sub-freezing temperatures as you drive to the gym for your morning workout. However, once you get to the gym and are running on the treadmill, you’ll probably find yourself quite warm. Nevertheless, through it all, it’s a sure thing that your body’s core temperature will have remained within the narrow range necessary for survival. This happens because in the heat, we sweat and our blood vessels dilate to allow more blood to flow close to the skin where it can be cooled and, conversely, in the cold, sweating stops, blood vessels constrict and we produce heat by shivering.
The homeostatic regulation of body temperature ensures that every human will register a similar value if they have their temperature taken. And homeostatic regulation of body fat levels would suggest the same. With this in mind, it is interesting to note that the animals we see on nature shows appear to conform to this model; for example, it’s rare to see a fat tiger or skinny elephant. However, when you walk down the street and observe your fellow humans, similar constancy will not be in effect. This suggests that humans exhibit non-homeostatic control of the regulation of energy balance, which forms the basis for Professor Borer’s article.
This article was originally published in New Living Magazine, which can be accessed on-line at www.newliving.com.
